User:Oleg Kovalevskiy/Sandbox 2

HlyIIR (Hemolysin II Regulator) is a transcriptional regulator of the gene for pore-forming toxin hemolysin II of Bacillus cereus.

Introduction
B. cereus is a Gram-positive, spore-forming bacterium, commonly found in soil or water. It is also found as a contaminant in food and pharmaceutical products. Some strains of this microorganism could caus emetic and enteric food-poisoning, periodontitis and systemic infections. The pathogenic properties of B. cereus are determined by the production of several extracellular virulence factors. Production of such toxins by bacterial cells is tightly controlled by different regulatory systems. In particular, synthesis of pore-forming cytotoxin hemolysin II is controlled at the level of transcription by HlyIIR.

Hemolysin II Regulator
The gene encoding HlyIIR (201 amino acid residues) is located immediately downstream of hemolysin II gene. The presence of HlyIIR decreases the level of hemolysin II expression in heterologous systems in vivo (Escherichia coli and B. subtilis). Purified HlyIIR protein also inhibited in vitro transcription from the promoter of hemolysin II gene. HlyIIR is a DNA-binding protein recognising a specific 44-bp region in hemolysin II gene promoter-operator segment. This segment is unusually organised, being a long perfect inverted repeat containing degenerate subrepeats. Two HlyIIR dimers non-cooperatively and independently bind to the operator with an apparent dissociation constant falling in a nanomolar range. Interestingly, mechanism of HlyIIR repression is not just a simple competition with RNA-polymerase (RNAP) for the promoter. HlyIIR forms a ternary complex with RNAP on the promoter of hemolysin II gene and represses transcription by inhibiting isomerisation of RNAP closed promoter complex into a catalytically active open promoter complex.